EP0063083B1 - X rays detector - Google Patents

X rays detector Download PDF

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Publication number
EP0063083B1
EP0063083B1 EP82400629A EP82400629A EP0063083B1 EP 0063083 B1 EP0063083 B1 EP 0063083B1 EP 82400629 A EP82400629 A EP 82400629A EP 82400629 A EP82400629 A EP 82400629A EP 0063083 B1 EP0063083 B1 EP 0063083B1
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Prior art keywords
chamber
electrodes
rays
ionization
auxiliary
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German (de)
French (fr)
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EP0063083A1 (en
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Robert Allemand
Jean-Jacques Gagelin
Edmond Tournier
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Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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Commissariat a lEnergie Atomique CEA
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J47/00—Tubes for determining the presence, intensity, density or energy of radiation or particles
    • H01J47/02—Ionisation chambers

Definitions

  • the present invention relates to an X-ray detector, in particular of X-rays which have passed through an object or an organ and which are supplied by a source emitting, towards the object or the organ, a plane beam of incident X-rays. with a wide angular opening and a small thickness.
  • This invention applies more particularly to the tomography of organs, but also to industrial control, such as baggage control for example.
  • X-ray detectors make it possible to measure the absorption of an X-ray beam passing through an object or an organ, this absorption being linked to the density of the tissues of the organ examined or the density of the materials constituting the object studied. Examples of detectors of this type are described, in particular, in patent application FR-A-2 314 699.
  • ionization X-ray detectors used in tomography are of the multicell type and include cells delimited by conductive plates perpendicular to the plane of the X-ray beam and brought alternately to positive and negative potentials. These cells are located in a sealed enclosure containing an ionizable gas.
  • the advantages of this type of multicellular detector are as follows: they provide good collimation of X-rays when the plates used in the detection cells are made of a very absorbent material; the collection time of the charges resulting from the ionization of the gas by X-rays is very short because of the small spacing of the conductive plates and the good separation between the detection cells.
  • this type of detector has significant drawbacks: it is possible to reduce the thickness of the plates in order to increase the quantity of X-rays detected, but to the detriment of collimation due to the small thickness of the plates; this small thickness of the plates also causes a very large microphone.
  • detectors of this type have a great complexity of production which leads to a high manufacturing cost and they require mounting in soil. dusted, because any dust on one of the plates can cause ignition or deterioration of the leakage current between two consecutive plates. It is added to these drawbacks that the numerous plates used require very numerous electrical connections, inside the sealed chamber, which poses difficult problems of reliability of the welds of the connections on the plates.
  • This other type of detector comprises a sealed chamber containing a gas ionizable by rays from the organ or object and, in this chamber, a plate for collecting the electrons resulting from the ionization of the gas; this plate is parallel to the plane of the beam of incident rays and it is brought to a positive high voltage.
  • a series of electrodes for collecting the ions resulting from the ionization of the gas by the X-rays coming from the object is arranged in parallel and facing the preceding plate; these ion collection electrodes are brought to a potential close to zero and are directed towards the source which emits the X-rays, in the direction of the object.
  • Each ion collection electrode defines an elementary cell of the detector. These electrodes are located in a plane parallel to the plane of the beam of the incident rays and respectively provide a current which is the sum, on the one hand of a measurement current proportional to the quantity of ions obtained by the ionization of the gas in sight of each electrode, under the effect of the rays coming from the object or the organ, in a direction corresponding to that of the incident rays and, on the other hand, of a diffusion current coming from the rays diffused by l 'object or by the organ, or in general by all the obstacles encountered by the incident rays, in directions other than that of the incident rays.
  • This type of detector has certain advantages: there are no longer, as in the detector mentioned above, separation plates; this eliminates any annoying phenomenon of microphony. Due to the removal of these separation plates, the quantity of X-rays detected is maximum; the realization of this type of detector is very simple and it is very little sensitive to dust.
  • this type of detector has a serious drawback which results from the fact that the current collected on each of the electrodes brought to a potential close to zero includes a parasitic current, which falsifies the measurements; this current is a diffusion current coming from rays scattered in other directions than that of the incident rays.
  • the object of the present invention is to remedy this drawback and in particular to produce an X-ray detector which makes it possible to eliminate, in the current collected on each of the electrodes which are brought to a potential close to zero, the parasitic current resulting from the rays scattered, in particular by the object or by the organ, in other directions than that of the incident rays.
  • the subject of the invention is an X-ray detector capable of detecting, for example, rays having passed through an object or an organ and being supplied by a source emitting, towards the object or the organ, a plane beam of rays. X incidents, this beam having a wide angular opening and a small thickness, this detector comprising at least one sealed main chamber containing at least one gas ionizable by X-rays and, in this chamber, a plate for collecting the charges resulting from the ionization of the gas, this plate being parallel to the plane of the beam of incident rays and being brought to a first potential, and a series of planar electrodes for collecting the charges resulting from the ionization of the gas, these electrodes being situated opposite the charge collection plate, in a plane parallel to the plane of the beam of incident rays on the side opposite to that on which the charge collection plate is located, each of them having its largest d dimension directed towards the source, defining an elementary detection cell and supplying a current which is the sum of a measurement
  • the electrodes for collecting the charges from the main ionization chamber are carried by one of the faces of an electrically insulating plate, the plate for collecting the charges from said main ionization chamber. being brought to a second determined potential, the auxiliary ionization chamber containing the same ionizable gas as the main ionization chamber and comprising a series of charge collection electrodes carried by the other face of the electrically insulating plate, these electrodes being respectively connected to the electrodes of the main ionization chamber and being brought to the same second potential close to zero, the charge collection plate of the auxiliary ionization chamber being parallel to the electrically insulating plate, located opposite the electrodes of electron collection and brought to a third potential of sign opposite to the first potential.
  • the charge collection plate of the main chamber and the charge collection plate of the auxiliary chamber are identical, the charge collection electrodes of the main chamber being respectively identical to the charge collection electrodes of the auxiliary chamber.
  • the electrically insulating plate supporting the series of electrodes of the main and auxiliary chambers is located midway between the charge collection plate of the main chamber and the charge collection plate of the auxiliary chamber.
  • the charge collection electrodes of the main chamber are respectively located opposite the charge collection electrodes of the auxiliary chamber.
  • the first and third potentials have the same absolute value.
  • the ionizable gas is xenon.
  • the electrodes for collecting ions and electrons from the main and auxiliary chambers consist of a deposit of copper on an insulating support.
  • FIG. 1 shows schematically and in perspective, a detector of known type comprising a plate 1 brought to a positive high voltage + HT and, opposite, a series of electrodes 2 brought to a potential close to zero volts.
  • This plate and these electrodes are located in a sealed main chamber 3, shown diagrammatically and which contains at least one ionizable gas, such as xenon for example.
  • This detector makes it possible to detect the X-rays which have passed through an object or an organ 0, these rays being supplied by a point source S which emits towards the object or the organ, a plane beam F of incident X-rays; this beam has a wide angular opening and a small thickness.
  • the plate 1 is parallel to the plane of the beam of incident rays, while the plane electrodes 2 are situated in a plane parallel to the plane of the beam of incident rays, opposite the plate 1.
  • the pressure of the xenon inside the sealed chamber has a value between 10 and 20 bars; this gas can also be added to other electropositive gases intended to improve detection.
  • the electrodes 2 form converging bands in the direction of the source S.
  • Figure 2 shows schematically a front view of the previous detector.
  • This figure shows the plate 1 brought to a positive potential + HT as well as the electrodes 2 brought to a potential close to zero volts; these electrodes are supported by an electrically insulating plate 4 and each of them is connected to an amplifier 5 which makes it possible to draw the current flowing in each of the electrodes; these currents are applied to a processing (not shown) and visualization system, which makes it possible to visualize the body or the object O crossed by the X-rays emitted by the source S.
  • dotted lines vertical, the field lines and, by horizontal dotted lines, the equipotentials of the electric field created by the potential difference between the positive plate 1 and the electrodes 2 brought to a potential close to zero.
  • Xe + represents the positive xenon ions which go towards the electrodes 2 and by e- the electrons which go to the plate 1, these ions and these electrons resulting from the xenon ionization by X-rays from the object or organ O.
  • FIG. 3 shows schematically and in perspective, a detector according to the invention.
  • This detector comprises a sealed chamber 6 containing at least one ionizable gas such as xenon for example; this chamber is subdivided into two ionization chambers: a main ionization chamber 3 and an auxiliary ionization chamber 7.
  • the main ionization chamber 3 contains, like the detector of the type known in FIG.
  • the electrodes 2 converge in the direction of the source S.
  • Each of the electrodes 2 of the main ionization chamber 3 is connected to an amplifier 5 which makes it possible to take samples, for processing; the current flowing in each of these electrodes.
  • the auxiliary ionization chamber 7 located outside the X-ray beam is attached to the main chamber to compensate for the diffusion current coming from the X-rays diffused by the organ O.
  • the electrodes 2 of the main ionization chamber 3 respectively supply a current which is the sum of a part, of a measurement current proportional to the quantity of ions obtained by the ionization of the gas next to each electrode of the main ionization chamber, under the effect rays coming from the object, in directions corresponding to that of the incident rays 9, and a diffusion current resulting from the ionization of the gas by the rays scattered 8 by the object, in directions other than that incident rays.
  • the auxiliary ionization chamber 7 contains, like the main ionization chamber, a plate 10 parallel to the plane of the incident X-ray beam, which is brought to a negative high voltage - HT, as well as a series of electrodes 11 planes, parallel to the plane of the incident X-ray beam, situated on another face of the insulating plate 4 which carries the electrodes 2 of the main ionization chamber 3.
  • These electrodes 11 are worn, like the electrodes 2 of the chamber d main ionization, at a potential close to zero. They are respectively connected by connections 12, to the corresponding electrodes of the main ionization chamber 3.
  • the electrodes 11 of the auxiliary ionization chamber and the electrodes 2 of the main ionization chamber are preferably identical and located next to each other.
  • the auxiliary ionization chamber 7 makes it possible, as will be seen below in detail, to compensate, for the subsequent treatment of the currents originating from the amplifiers 5, the parasitic currents which circulate in each electrode of the main ionization chamber and which originate X-rays scattered by the object or organ 0.
  • the electrodes 11 of the auxiliary ionization chamber 7 are electrodes for collecting electrons e-, while plate 10 is a plate for collecting Xe + ions from of the xenon ionization contained in the auxiliary chamber 7, by the X-rays scattered by the object or the organ O.
  • the electrodes of the auxiliary ionization chamber are located opposite the electrodes of the chamber main ionization and positive and negative high voltages have the same absolute value.
  • FIG. 4 schematically represents a side view of the detector of the invention.
  • this view there is a point source S, the object or the organ 0, one of the rays 9 emitted by the source S and, at the output of the object 0, the direct ray 13 coming from the object 0, in the same direction as the incident ray 9; one also distinguishes in this figure one of the scattered rays 8, coming from the object 0, in a direction different from the direction of the incident ray 9.
  • one shows one of the electrodes 2 of the chamber d main ionization which is connected to an amplifier 5 and which is brought to a potential close to zero and, one of the electrodes 11 of the auxiliary ionization chamber 7, which is located opposite the electrode 2 and which is separated from this electrode by the insulating plate 4.
  • the connection 12 between the electrodes of the main and auxiliary ionization chambers has also been shown.
  • the plates 1 and 10 of the main and auxiliary ionization chambers brought respectively to positive and negative potentials + HT and - HT.
  • the sealed chamber 6 which contains the ionizable gas has not been shown in detail; the insulating plates 15, 14 support the conductive plates 1, 10 of the main and auxiliary ionization chambers.
  • the ionizable gas is for example xenon
  • This ionization is represented schematically in the figure, by Xe + ions which are attracted by the electrodes 2, and by e- electrons which are attracted by the positive plate 1.
  • An ionization thus occurs opposite each of the electrodes of the main ionization chamber using X-rays from the object, in the direction of the incident rays.
  • These ion movements produce respectively in each electrode, a current 1 which is the sum of a current I M , resulting from the ionization of the gas opposite each of the electrodes, under the effect of X-rays from the object (rays represented at 13 in the figure), in a direction corresponding to that of the incident rays, and of a so-called diffusion current I D , which results from the ionization of the gas, opposite each of the electrodes from the rays scattered by the object (represented at 8 in the figure) or by all the material obstacles encountered by the incident X-rays, in directions which do not correspond to those of the incident X-rays.
  • the ionization chamber 7 makes it possible to compensate for this "diffusion current", thanks to the ionization produced in this chamber by the scattered X-rays 8; this ionization causes the circulation, in the electrodes 11 of the auxiliary chamber, of a current 1 D which is cut off, thanks to the connection 12, from the “parasitic diffusion current taken into account by the electrodes of the ionization chamber main.
  • the study demonstrated that the current collected in the auxiliary ionization chamber was representative of the diffusion current collected in the main ionization chamber.
  • the amplifiers 5 connected to each of the electrodes of the main and auxiliary ionization chambers receive a current l m which is effectively the measurement current corresponding to the ionization of the gas, caused opposite each of the electrodes of the chamber d main ionization by the rays 13 coming from the object or the organ in the directions which correspond to those of the incident rays 9.
  • the plates and electrodes of the main and auxiliary ionization chambers are preferably produced in the form of a copper deposit on an insulating support.
  • the number of cells in each chamber can be greater than 500, for an opening angle of the X-ray beam greater than 40 °; in this case, the pitch between each of the electrodes of each chamber is approximately 1 mm.
  • the insulating plate 4 which supports the electrodes of the main and auxiliary chambers is located halfway between these plates 1 and 10, respectively brought to the positive and negative potential. The distance between these plates 1 and 10 is approximately 14 mm and the ion collection time is close to 10 ms.

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Description

La présente invention concerne un détecteur de rayons X, notamment de rayons X qui ont traversé un objet ou un organe et qui sont fournis par une source émettant, en direction de l'objet ou de l'organe, un faisceau plan de rayons X incidents présentant une large ouverture angulaire et une faible épaisseur. Cette invention s'applique plus particulièrement à la tomographie d'organes, mais également au contrôle industriel, tel que le contrôle de bagages par exemple.The present invention relates to an X-ray detector, in particular of X-rays which have passed through an object or an organ and which are supplied by a source emitting, towards the object or the organ, a plane beam of incident X-rays. with a wide angular opening and a small thickness. This invention applies more particularly to the tomography of organs, but also to industrial control, such as baggage control for example.

Ces détecteurs de rayons X permettent de mesurer l'absorption d'un faisceau de rayons X traversant un objet ou un organe, cette absorption étant liée à la densité des tissus de l'organe examiné ou la densité des matériaux constituant l'objet étudié. Des exemples de détecteurs de ce type sont décrits, en particulier, dans la demande de brevet FR-A-2 314 699.These X-ray detectors make it possible to measure the absorption of an X-ray beam passing through an object or an organ, this absorption being linked to the density of the tissues of the organ examined or the density of the materials constituting the object studied. Examples of detectors of this type are described, in particular, in patent application FR-A-2 314 699.

.Si l'on veut établir la carte de densité d'un organe ou d'un objet, il est possible et connu d'envoyer un faisceau plan de rayons X incidents sur cet objet ou cet organe, ce faisceau présentant une large ouverture angulaire et une faible épaisseur et d'observer pour chaque position des faisceaux de rayons X incidents par rapport à l'objet ou l'organe, l'absorption correspondante. Une multiplicité de balayages dans des directions croisées, permet de connaître grâce au détecteur de rayons X, après un traitement numérique approprié des signaux recueillis sur les cellules du détecteur, la valeur de l'absorption des rayons X en un point du plan de coupe considéré, et ainsi de connaître la densité des tissus de l'organe ou la densité des matériaux constituant l'objet..If you want to establish the density map of an organ or an object, it is possible and known to send a plane beam of X-rays incident on this object or this organ, this beam having a wide angular opening and a small thickness and observe for each position of the incident X-ray beams with respect to the object or the organ, the corresponding absorption. A multiplicity of scans in crossed directions, makes it possible to know thanks to the X-ray detector, after an appropriate digital processing of the signals collected on the cells of the detector, the value of the absorption of X-rays at a point of the considered section plane , and thus to know the density of the tissues of the organ or the density of the materials constituting the object.

La plupart des détecteurs de rayons X à ionisation, utilisés en tomographie sont de type multicellulaire et comportent des cellules délimitées par des plaques conductrices perpendiculaires au plan du faisceau de rayons X et portées alternativement à des potentiels positifs et négatifs. Ces cellules sont situées dans une enceinte étanche contenant un gaz ionisable. Les avantages de ce type de détecteur multicellulaires, sont les suivants : ils procurent une bonne collimation des rayons X lorsque les plaques utilisées dans les cellules de détection sont constituées dans un matériau très absorbant ; le temps de collection des charges résultant de l'ionisation du gaz par les rayons X est très faible à cause du faible espacement des plaques conductrices et de la bonne séparation entre les cellules de détection. Cependant, ce type de détecteur présente des inconvénients importants : il est possible de diminuer l'épaisseur des plaques afin d'augmenter la quantité de rayons X détectés, mais au détriment de la collimation du fait de la faible épaisseur des plaques ; cette faible épaisseur des plaques provoque en outre une microphonie très importante. Enfin, les détecteurs de ce type présentent une grande complexité de réalisation qui entraîne un coût de fabrication élevé et ils nécessitent un montage en salie. dépoussiérée, car toute poussière sur l'une des plaques, peut provoquer un amorçage ou une détérioration du courant de fuite entre deux plaques consécutives. Il s'ajoute à ces inconvénients.que les nombreuses plaques utilisées nécessitent des connexions électriques très nombreuses, à l'intérieur de la chambre étanche, ce qui pose des problèmes difficiles de fiabilité des soudures des connexions sur les plaques.Most of the ionization X-ray detectors used in tomography are of the multicell type and include cells delimited by conductive plates perpendicular to the plane of the X-ray beam and brought alternately to positive and negative potentials. These cells are located in a sealed enclosure containing an ionizable gas. The advantages of this type of multicellular detector are as follows: they provide good collimation of X-rays when the plates used in the detection cells are made of a very absorbent material; the collection time of the charges resulting from the ionization of the gas by X-rays is very short because of the small spacing of the conductive plates and the good separation between the detection cells. However, this type of detector has significant drawbacks: it is possible to reduce the thickness of the plates in order to increase the quantity of X-rays detected, but to the detriment of collimation due to the small thickness of the plates; this small thickness of the plates also causes a very large microphone. Finally, detectors of this type have a great complexity of production which leads to a high manufacturing cost and they require mounting in soil. dusted, because any dust on one of the plates can cause ignition or deterioration of the leakage current between two consecutive plates. It is added to these drawbacks that the numerous plates used require very numerous electrical connections, inside the sealed chamber, which poses difficult problems of reliability of the welds of the connections on the plates.

On connaît un autre type de détecteur qui présente une structure beaucoup plus simple, mais qui n'est pas parfait. Cet autre type de détecteur comprend une chambre étanche contenant un gaz ionisable par des rayons issus de l'organe ou de l'objet et, dans cette chambre, une plaque de collection des électrons résultant de l'ionisation du gaz ; cette plaque est parallèle au plan du faisceau de rayons incidents et elle est portée à une haute tension positive. Une série d'électrodes de collection des ions résultant de l'ionisation du gaz par les rayons X issus de l'objet, est disposée parallèlement et en regard de la plaque précédente ; ces électrodes de collection des ions sont portées à un potentiel voisin de zéro et sont dirigées vers la source qui émet les rayons X, en direction de l'objet. Chaque électrode de collection des ions définit une cellule élémentaire du détecteur. Ces électrodes sont situées dans un plan parallèle au plan du faisceau des rayons incidents et fournissent respectivement un courant qui est la somme, d'une part d'un courant de mesure proportionnel à la quantité d'ions obtenus par l'ionisation du gaz en regard de chaque électrode, sous l'effet des rayons issus de l'objet ou de l'organe, dans une direction correspondant à celle des rayons incidents et, d'autre part, d'un courant de diffusion provenant des rayons diffusés par l'objet ou par l'organe, ou de manière générale par tous les obstacles rencontrés par les rayons incidents, dans d'autres directions que celle des rayons incidents.Another type of detector is known which has a much simpler structure, but which is not perfect. This other type of detector comprises a sealed chamber containing a gas ionizable by rays from the organ or object and, in this chamber, a plate for collecting the electrons resulting from the ionization of the gas; this plate is parallel to the plane of the beam of incident rays and it is brought to a positive high voltage. A series of electrodes for collecting the ions resulting from the ionization of the gas by the X-rays coming from the object, is arranged in parallel and facing the preceding plate; these ion collection electrodes are brought to a potential close to zero and are directed towards the source which emits the X-rays, in the direction of the object. Each ion collection electrode defines an elementary cell of the detector. These electrodes are located in a plane parallel to the plane of the beam of the incident rays and respectively provide a current which is the sum, on the one hand of a measurement current proportional to the quantity of ions obtained by the ionization of the gas in sight of each electrode, under the effect of the rays coming from the object or the organ, in a direction corresponding to that of the incident rays and, on the other hand, of a diffusion current coming from the rays diffused by l 'object or by the organ, or in general by all the obstacles encountered by the incident rays, in directions other than that of the incident rays.

Ce type de détecteur présente certains avantages : il n'y a plus, comme dans le détecteur mentionné précédemment, de plaques de séparation ; ceci élimine tout phénomène gênant de microphonie. Du fait de la suppression de ces plaques de séparation, la quantité de rayons X détectés est maximale ; la réalisation de ce type de détecteur est très simple et il est très peu sensible aux poussières. Ce type de détecteur présente cependant un grave inconvénient qui résulte du fait que le courant recueilli sur chacune des électrodes portées à un potentiel voisin de zéro, comprend un courant parasite, qui fausse les mesures ; ce courant est un courant de diffusion provenant des rayons diffusés dans d'autres directions que celle des rayons incidents.This type of detector has certain advantages: there are no longer, as in the detector mentioned above, separation plates; this eliminates any annoying phenomenon of microphony. Due to the removal of these separation plates, the quantity of X-rays detected is maximum; the realization of this type of detector is very simple and it is very little sensitive to dust. However, this type of detector has a serious drawback which results from the fact that the current collected on each of the electrodes brought to a potential close to zero includes a parasitic current, which falsifies the measurements; this current is a diffusion current coming from rays scattered in other directions than that of the incident rays.

La présente invention a pour but de remédier à cet inconvénient et notamment de réaliser un détecteur de rayons X qui permet d'éliminer, dans le courant recueilli sur chacune des électrodes qui sont portées à un potentiel voisin de zéro, le courant parasite résultant des rayons diffusés, notamment par l'objet ou par l'organe, dans d'autres directions que celle des rayons incidents.The object of the present invention is to remedy this drawback and in particular to produce an X-ray detector which makes it possible to eliminate, in the current collected on each of the electrodes which are brought to a potential close to zero, the parasitic current resulting from the rays scattered, in particular by the object or by the organ, in other directions than that of the incident rays.

L'invention a pour objet un détecteur de rayons X aptes à détecter par exemple des rayons ayant traversé un objet ou un organe et étant fournis par une source émettant, en direction de l'objet ou de l'organe, un faisceau plan de rayons X incidents, ce faisceau présentant une large ouverture angulaire et une faible épaisseur, ce détecteur comprenant au moins une chambre-principale étanche contenant au moins un gaz ionisable par les rayons X et, dans cette chambre, une plaque de collection des charges résultant de l'ionisation du gaz, cette plaque étant parallèle au plan du faisceau de rayons incidents et étant portée à un premier potentiel, et une série d'électrodes planes de collection des charges résultant de l'ionisation du gaz, ces électrodes étant situées en regard de la plaque de collection des charges, dans un plan parallèle au plan du faisceau de rayons incidents du côté opposé à celui où se trouve la plaque de collection des charges, chacune d'entre elles ayant sa plus grande dimension dirigée vers la source, définissant une cellule élémentaire de détection et fournissant un courant qui est la somme d'un courant de mesure et d'un courant de diffusion, respectivement proportionnels aux quantités de charges obtenues par l'ionisation du gaz en regard sous l'effet des rayons issus de l'objet dans des directions correspondant à celles des rayons incidents, d'une part, et sous l'effet des rayons diffusés dans d'autres directions que celles des rayons incidents, d'autre part, et ce détecteur étant caractérisé en ce qu'il comprend une chambre d'ionisation auxiliaire accolée à la chambre d'ionisation principale, entièrement située d'un côté du plan du' faisceau de rayons incidents et ne recevant que lesdits rayons diffusés dans d'autres directions que celles desdits rayons incidents, laquelle comporte une plaque conductrice dans un premier plan parallèle à celui du faisceau ainsi que des électrodes situées dans un second plan également parallèle à celui du faisceau et dont chacune définit une cellule élémentaire de détection fournissant un courant de diffusion, exclusivement, en ce qu'à chaque cellule élémentaire de détection de la chambre d'ionisation principale est associée une cellule élémentaire de détection de la chambre de diffusion auxiliaire et en ce que le courant de diffusion fourni par cette dernière cellule de détection sert à compenser, dans la portion du circuit où s'effectue la mesure, le courant de diffusion de ladite cellule élémentaire de détection de la chambre principale qui lui est associée.The subject of the invention is an X-ray detector capable of detecting, for example, rays having passed through an object or an organ and being supplied by a source emitting, towards the object or the organ, a plane beam of rays. X incidents, this beam having a wide angular opening and a small thickness, this detector comprising at least one sealed main chamber containing at least one gas ionizable by X-rays and, in this chamber, a plate for collecting the charges resulting from the ionization of the gas, this plate being parallel to the plane of the beam of incident rays and being brought to a first potential, and a series of planar electrodes for collecting the charges resulting from the ionization of the gas, these electrodes being situated opposite the charge collection plate, in a plane parallel to the plane of the beam of incident rays on the side opposite to that on which the charge collection plate is located, each of them having its largest d dimension directed towards the source, defining an elementary detection cell and supplying a current which is the sum of a measurement current and a diffusion current, respectively proportional to the quantities of charges obtained by the ionization of the gas opposite under the effect of rays from the object in directions corresponding to those of incident rays, on the one hand, and under the effect of rays scattered in directions other than those of incident rays, on the other hand, and this detector being characterized in that it comprises an auxiliary ionization chamber attached to the main ionization chamber, entirely situated on one side of the plane of the 'beam of incident rays and receiving only said rays scattered in other directions than those of said incident rays, which comprises a conductive plate in a first plane parallel to that of the beam as well as electrodes located in a second plane also parallel to that of the beam e t each of which defines an elementary detection cell supplying a diffusion current, exclusively, in that to each elementary detection cell of the main ionization chamber is associated an elementary detection cell of the auxiliary diffusion chamber and in that that the diffusion current supplied by this latter detection cell serves to compensate, in the portion of the circuit where the measurement is made, the diffusion current from said elementary detection cell of the main chamber which is associated with it.

Selon une autre caractéristique de l'invention, les électrodes de collection des charges de la chambre d'ionisation principale sont portées par l'une des faces d'une plaque électriquement isolante, la ptaque de collection des charges de ladite chambre d'ionisation principale étant portée à un second potentiel déterminé, la chambre d'ionisation auxiliaire contenant le même gaz ionisable que la chambre d'ionisation principale et comportant une série d'électrodes de collection des charges portées par l'autre face de la plaque électriquement isolante, ces électrodes étant respectivement reliées aux électrodes de la chambre d'ionisation principale et étant portées au même second potentiel voisin de zéro, la plaque de collection des charges de la chambre d'ionisation auxiliaire étant parallèle à la plaque électriquement isolante, située en regard des électrodes de collection des électrons et portée à un troisième potentiel de signe opposé au premier potentiel.According to another characteristic of the invention, the electrodes for collecting the charges from the main ionization chamber are carried by one of the faces of an electrically insulating plate, the plate for collecting the charges from said main ionization chamber. being brought to a second determined potential, the auxiliary ionization chamber containing the same ionizable gas as the main ionization chamber and comprising a series of charge collection electrodes carried by the other face of the electrically insulating plate, these electrodes being respectively connected to the electrodes of the main ionization chamber and being brought to the same second potential close to zero, the charge collection plate of the auxiliary ionization chamber being parallel to the electrically insulating plate, located opposite the electrodes of electron collection and brought to a third potential of sign opposite to the first potential.

Selon une autre caractéristique, la plaque de collection des charges de la chambre principale et la plaque de collection des charges de la chambre auxiliaire sont identiques, les électrodes de collection des charges de la chambre principale étant respectivement identiques aux électrodes de collection des charges de la chambre auxiliaire.According to another characteristic, the charge collection plate of the main chamber and the charge collection plate of the auxiliary chamber are identical, the charge collection electrodes of the main chamber being respectively identical to the charge collection electrodes of the auxiliary chamber.

Selon une autre caractéristique, la plaque électriquement isolante supportant les séries d'électrodes des chambres principale et auxiliaire, est située à mi-distance entre la plaque de collection des charges de la chambre principale et la plaque de collection des charges de la chambre auxiliaire.According to another characteristic, the electrically insulating plate supporting the series of electrodes of the main and auxiliary chambers is located midway between the charge collection plate of the main chamber and the charge collection plate of the auxiliary chamber.

Selon une autre caractéristique, les électrodes de collection des charges de la chambre principale sont respectivement situées en regard des électrodes de collection des charges de la chambre auxiliaire.According to another characteristic, the charge collection electrodes of the main chamber are respectively located opposite the charge collection electrodes of the auxiliary chamber.

Selon une autre caractéristique, les premier et troisième potentiels ont la même valeur absolue.According to another characteristic, the first and third potentials have the same absolute value.

Selon une autre caractéristique, le gaz ionisable est du xénon.According to another characteristic, the ionizable gas is xenon.

Selon une autre caractéristique, les électrodes de collection des ions et des électrons des chambres principale et auxiliaire sont constituées par un dépôt de cuivre sur un support isolant.According to another characteristic, the electrodes for collecting ions and electrons from the main and auxiliary chambers consist of a deposit of copper on an insulating support.

D'autres caractéristiques et avantages de l'invention ressortiront mieux de la description qui va suivre, donnée en référence aux dessins annexés dans lesquels :

  • la figure 1 représente schématiquement et en perspective, un détecteur de type connu comprenant une plaque portée à un potentiel positif et, en regard, une série d'électrodes portées à un potentiel voisin de zéro ;
  • la figure 2 représente schématiquement une vue de face du détecteur précédent ;
  • la figure 3 représente schématiquement et en perspective, un détecteur conforme à l'invention.
  • la figure 4 représente schématiquement une vue latérale du détecteur selon l'invention.
Other characteristics and advantages of the invention will emerge more clearly from the description which follows, given with reference to the appended drawings in which:
  • Figure 1 shows schematically and in perspective, a known type of detector comprising a plate brought to a positive potential and, opposite, a series of electrodes brought to a potential close to zero;
  • Figure 2 schematically shows a front view of the previous detector;
  • Figure 3 shows schematically and in perspective, a detector according to the invention.
  • FIG. 4 schematically represents a side view of the detector according to the invention.

La figure 1 représente schématiquement et en perspective, un détecteur de type connu comprenant une plaque 1 portée à une haute tension positive + HT et, en regard, une série d'électrodes 2 portées à un potentiel voisin de zéro volt. Cette plaque et ces électrodes sont situées dans une chambre principale 3 étanche, représentée schématiquement et qui contient au moins un gaz ionisable, tel que le xénon par exemple. Ce détecteur permet de détecter les rayons X qui ont traversé un objet ou un organe 0, ces rayons étant fournis par une source S ponctuelle qui émet en direction de l'objet ou de l'organe, un faisceau F plan de rayons X incidents ; ce faisceau présente une large ouverture angulaire et une faible épaisseur. La plaque 1 est parallèle au plan du faisceau de rayons incidents, tandis que les électrodes planes 2 sont situées dans un plan parallèle au plan du faisceau de rayons incidents, en regard de la plaque 1. La plaque 1, qui est portée à un potentiel positif voisin de quelques kilovolts, est une plaque de collection des électrons, tandis que les électrodes 2 sont des électrodes de collection des ions. Ces électrodes sont généralement portées par une plaque isolante (non représentée sur cette figure) et sont isolées électriquement entre elles. La pression du xénon à l'intérieur de la chambre étanche a une valeur comprise entre 10 et 20 bars ; ce gaz peut d'ailleurs être additionné à d'autres gaz électropositifs destinés à améliorer la détection. Les électrodes 2 forment des bandes convergentes en direction de la source S.FIG. 1 shows schematically and in perspective, a detector of known type comprising a plate 1 brought to a positive high voltage + HT and, opposite, a series of electrodes 2 brought to a potential close to zero volts. This plate and these electrodes are located in a sealed main chamber 3, shown diagrammatically and which contains at least one ionizable gas, such as xenon for example. This detector makes it possible to detect the X-rays which have passed through an object or an organ 0, these rays being supplied by a point source S which emits towards the object or the organ, a plane beam F of incident X-rays; this beam has a wide angular opening and a small thickness. The plate 1 is parallel to the plane of the beam of incident rays, while the plane electrodes 2 are situated in a plane parallel to the plane of the beam of incident rays, opposite the plate 1. The plate 1, which is brought to a potential positive close to a few kilovolts, is an electron collection plate, while the electrodes 2 are ion collection electrodes. These electrodes are generally carried by an insulating plate (not shown in this figure) and are electrically isolated from each other. The pressure of the xenon inside the sealed chamber has a value between 10 and 20 bars; this gas can also be added to other electropositive gases intended to improve detection. The electrodes 2 form converging bands in the direction of the source S.

Le fonctionnement de la chambre 3 est le suivant :

  • Lorsqu'un photon X arrive dans cette chambre 3 contenant un gaz, il va interagir avec une ou plusieurs molécules de ce gaz.
The operation of chamber 3 is as follows:
  • When a photon X arrives in this chamber 3 containing a gas, it will interact with one or more molecules of this gas.

Si l'énergie (Ex) de ce photon X est supérieure à l'énergie d'ionisation du gaz (21,6 eV pour le Xénon), il va ioniser les molécules de gaz sur son trajet : par exemple, si Ex = 80 keV dans le Xe, le nombre de molécules de Xénon ionisées est N = 80 000/21,6 = 3 700.If the energy (Ex) of this photon X is greater than the ionization energy of the gas (21.6 eV for Xenon), it will ionize the gas molecules on its path: for example, if Ex = 80 keV in Xe, the number of ionized Xenon molecules is N = 80,000 / 21.6 = 3,700.

Il y a donc création de 3 700 Xe+ et de 3 700 éThere is therefore creation of 3,700 Xe + and 3,700 é

En l'absence de champ électrique, les particules précédentes se recombinent. Mais lorsque la' haute tension est appliquée, sous l'effet du champ électrique, ces particules chargées se séparent :

  • - les électrons e- se dirigent vers la plaque 1 à haute tension + HT,
  • - les ions Xe+ se déplacent vers l'électrode de mesure 2 (à OV).
In the absence of an electric field, the preceding particles recombine. But when high voltage is applied, under the effect of the electric field, these charged particles separate:
  • - the electrons e- go to plate 1 at high voltage + HT,
  • - the Xe + ions move towards the measurement electrode 2 (at OV).

C'est le déplacement d'une particule chargée à proximité, qui induit entre l'électrode de mesure 2 et l'électrode 1 à haute tension un courant lM qui peut être amplifié et mesuré. Ce courant est donc proportionnel au nombre de particules créées, soit par conséquent à l'énergie Ex du photon X incident.It is the displacement of a charged particle nearby, which induces between the measuring electrode 2 and the high voltage electrode 1 a current 1M which can be amplified and measured. This current is therefore proportional to the number of particles created, ie consequently to the energy Ex of the incident photon X.

On peut également remarquer que l'addition d'un gaz électronégatif dans la chambre 3 ne perturbe que le temps de collection des charges, à l'exclusion de leur nombre, car :

  • - les ions Xe+ se déplacent vers l'électrode de mesure 2, mais sont ralentis par les molécules de gaz électronégatifs se déplaçant en sens inverse,
  • - les quelques électrons restés libres se dirigent très rapidement (comme dans le gaz pur, environ 1 000 fois plus vite que les ions Xe+) vers l'électrode 1 à haute tension positive,
  • - les électrons, captés par les molécules électronégatives, entraînent ces molécules vers l'électrode 1 à haute tension avec une vitesse du même ordre de grandeur que celle des ions Xe+.
It can also be noted that the addition of an electronegative gas in chamber 3 only disturbs the collection time of the charges, excluding their number, because:
  • - the Xe + ions move towards the measurement electrode 2, but are slowed down by the electronegative gas molecules moving in opposite direction,
  • - the few remaining electrons go very quickly (as in pure gas, about 1000 times faster than Xe + ions) towards electrode 1 with high positive voltage,
  • - the electrons, captured by the electronegative molecules, entrain these molecules towards the high voltage electrode 1 with a speed of the same order of magnitude as that of the Xe + ions .

La figure 2 représente schématiquement une vue de face du détecteur précédent. On a représenté sur cette figure, la plaque 1 portée à un potentiel positif + HT ainsi que les électrodes 2 portées à un potentiel voisin de zéro volt ; ces électrodes sont supportées par une plaque électriquement isolante 4 et chacune d'elles est reliée à un amplificateur 5 qui permet de prélever le courant circulant dans chacune des électrodes ; ces courants sont appliqués à un système de traitement (non représenté) et de visualisation, qui permet de visualiser le corps ou l'objet O traversé par les rayons X émis par la source S. Sur cette figure, on a représenté par des lignes pointillées verticales, les lignes de champ et, par des lignes pointillées horizontales, les équipotentielles du champ électrique créé par la différence de potentiel entre la plaque positive 1 et les électrodes 2 portées à un potentiel voisin de zéro. Dans la chambre contenant au moins du xénon, on a représenté par Xe+ les ions positifs de xénon qui se dirigent vers les électrodes 2 et par e- les électrons qui se dirigent vers la plaque 1, ces ions et ces électrons résultant de l'ionisation du xénon par les rayons X issus de l'objet ou de l'organe O.Figure 2 shows schematically a front view of the previous detector. This figure shows the plate 1 brought to a positive potential + HT as well as the electrodes 2 brought to a potential close to zero volts; these electrodes are supported by an electrically insulating plate 4 and each of them is connected to an amplifier 5 which makes it possible to draw the current flowing in each of the electrodes; these currents are applied to a processing (not shown) and visualization system, which makes it possible to visualize the body or the object O crossed by the X-rays emitted by the source S. In this figure, we have shown by dotted lines vertical, the field lines and, by horizontal dotted lines, the equipotentials of the electric field created by the potential difference between the positive plate 1 and the electrodes 2 brought to a potential close to zero. In the chamber containing at least xenon, Xe + represents the positive xenon ions which go towards the electrodes 2 and by e- the electrons which go to the plate 1, these ions and these electrons resulting from the xenon ionization by X-rays from the object or organ O.

La figure 3 représente schématiquement et en perspective, un détecteur conforme à l'invention. Ce détecteur comprend une chambre étanche 6 contenant au moins un gaz ionisable tel que le xénon par exemple ; cette chambre se subdivise en deux chambres d'ionisation : une chambre d'ionisation principale 3 et une chambre d'ionisation auxiliaire 7. La chambre d'ionisation principale 3 contient comme le détecteur de type connu de la figure 1, une plaque 1 portée à une haute tension positive + HT et une série d'électrodes 2 portées à un potentiel voisin de zéro voit ; comme précédemment, ces électrodes sont planes et sont portées par une plaque 4 électriquement isolante ; la plaque 1 ainsi que les électrodes 2 sont situées dans un plan parallèle au plan du faisceau de rayons X issus de l'objet 0 (ce faisceau étant incomplètement représenté sur la figure). Les électrodes 2 convergent dans la direction de la source S. Chacune des électrodes 2 de la chambre d'ionisation principale 3 est reliée à un amplificateur 5 qui permet de prélever, en vue du traitement; le courant circulant dans chacune de ces électrodes. Selon l'invention, la chambre d'ionisation auxiliaire 7 située en dehors du faisceau de rayons X, est accolée à la chambre principale pour compenser le courant de diffusion provenant des rayons X diffusés par l'organe O. En effet, comme on le verra plus loin en détail, les électrodes 2 de la chambre d'ionisation principale 3, fournissent respectivement un courant qui est la somme d'une part, d'un courant de mesure proportionnel à la quantité d'ions obtenus par l'ionisation du gaz en regard de chaque électrode de la chambre d'ionisation principale, sous l'effet des rayons issus de l'objet, dans des directions correspondant à celle des rayons incidents 9, et d'un courant de diffusion résultant de l'ionisation du gaz par les rayons diffusés 8 par l'objet, dans d'autres directions que celle des rayons incidents. La chambre d'ionisation auxiliaire 7 contient, comme la chambre d'ionisation principale, une plaque 10 parallèle au plan du faisceau de rayons X incidents, qui est portée à une haute tension négative - HT, ainsi qu'une série d'électrodes 11 planes, parallèles au plan du faisceau de rayons X incidents, situées sur une autre face de la plaque isolante 4 qui porte les électrodes 2 de la chambre d'ionisation principale 3. Ces électrodes 11 sont portées, comme les électrodes 2 de la chambre d'ionisation principale, à un potentiel voisin de zéro. Elles sont respectivement reliées par des connexions 12, aux électrodes correspondantes de la chambre d'ionisation principale 3. Les électrodes 11 de la chambre d'ionisation auxiliaire et les électrodes 2 de la chambre d'ionisation principale sont, de préférence, identiques et situées en regard les unes des autres. La chambre d'ionisation auxiliaire 7 permet, comme on le verra plus loin en détail, de compenser, pour le traitement ultérieur des courants issus des amplificateurs 5, les courants parasites qui circulent dans chaque électrode de la chambre d'ionisation principale et qui proviennent des rayons X diffusés par l'objet ou l'organe 0. Les électrodes 11 de la chambre d'ionisation auxiliaire 7 sont des électrodes de collection des électrons e-, tandis que la plaque 10 est une plaque de collection des ions Xe+ provenant de l'ionisation du xénon contenu dans la chambre auxiliaire 7, par les rayons X diffusés par l'objet ou l'organe O. De préférence, les électrodes de la chambre d'ionisation auxiliaire, sont situées en regard des électrodes de la chambre d'ionisation principale et les hautes tensions positive et négative ont la même valeur absolue.Figure 3 shows schematically and in perspective, a detector according to the invention. This detector comprises a sealed chamber 6 containing at least one ionizable gas such as xenon for example; this chamber is subdivided into two ionization chambers: a main ionization chamber 3 and an auxiliary ionization chamber 7. The main ionization chamber 3 contains, like the detector of the type known in FIG. 1, a plate 1 carried at a positive high voltage + HT and a series of electrodes 2 brought to a potential close to zero sees; as before, these electrodes are planar and are carried by an electrically insulating plate 4; the plate 1 and the electrodes 2 are located in a plane parallel to the plane of the X-ray beam from the object 0 (this beam being incompletely shown in the figure). The electrodes 2 converge in the direction of the source S. Each of the electrodes 2 of the main ionization chamber 3 is connected to an amplifier 5 which makes it possible to take samples, for processing; the current flowing in each of these electrodes. According to the invention, the auxiliary ionization chamber 7 located outside the X-ray beam, is attached to the main chamber to compensate for the diffusion current coming from the X-rays diffused by the organ O. Indeed, as is will see further in detail, the electrodes 2 of the main ionization chamber 3, respectively supply a current which is the sum of a part, of a measurement current proportional to the quantity of ions obtained by the ionization of the gas next to each electrode of the main ionization chamber, under the effect rays coming from the object, in directions corresponding to that of the incident rays 9, and a diffusion current resulting from the ionization of the gas by the rays scattered 8 by the object, in directions other than that incident rays. The auxiliary ionization chamber 7 contains, like the main ionization chamber, a plate 10 parallel to the plane of the incident X-ray beam, which is brought to a negative high voltage - HT, as well as a series of electrodes 11 planes, parallel to the plane of the incident X-ray beam, situated on another face of the insulating plate 4 which carries the electrodes 2 of the main ionization chamber 3. These electrodes 11 are worn, like the electrodes 2 of the chamber d main ionization, at a potential close to zero. They are respectively connected by connections 12, to the corresponding electrodes of the main ionization chamber 3. The electrodes 11 of the auxiliary ionization chamber and the electrodes 2 of the main ionization chamber are preferably identical and located next to each other. The auxiliary ionization chamber 7 makes it possible, as will be seen below in detail, to compensate, for the subsequent treatment of the currents originating from the amplifiers 5, the parasitic currents which circulate in each electrode of the main ionization chamber and which originate X-rays scattered by the object or organ 0. The electrodes 11 of the auxiliary ionization chamber 7 are electrodes for collecting electrons e-, while plate 10 is a plate for collecting Xe + ions from of the xenon ionization contained in the auxiliary chamber 7, by the X-rays scattered by the object or the organ O. Preferably, the electrodes of the auxiliary ionization chamber are located opposite the electrodes of the chamber main ionization and positive and negative high voltages have the same absolute value.

La figure 4 représente schématiquement une vue latérale du détecteur de l'invention. Sur cette vue, on distingue la source ponctuelle S, l'objet ou l'organe 0, l'un des rayons 9 émis par la source S et, en sortie de l'objet 0, le rayon direct 13 issu de l'objet 0, dans la même direction que le rayon incident 9 ; on distingue aussi sur cette figure l'un des rayons diffusés 8, issu de l'objet 0, dans une direction différente de la direction du rayon incident 9. Sur la figure, on a représenté l'une des électrodes 2 de la chambre d'ionisation principale qui est reliée à un amplificateur 5 et qui est portée à un potentiel voisin de zéro et, l'une des électrodes 11 de la chambre d'ionisation auxiliaire 7, qui est située en regard de l'électrode 2 et qui est séparée de cette électrode par la plaque isolante 4. On a également représenté la connexion 12 entre les électrodes des chambres d'ionisation principale et auxiliaire. Enfin, on a représenté les plaques 1 et 10 des chambres d'ionisation principale et auxiliaire, portées respectivement à des potentiels positif et négatif + HT et - HT. Sur cette figure, on n'a pas représenté en détail lachambre étanche 6 qui contient le gaz ionisable ; les plaques isolantes 15, 14 supportent les plaques conductrices 1, 10 des chambres d'ionisation principale et auxiliaire. Lorsque le gaz ionisable est par exemple du xénon, les rayons X représentés en 13 et qui sont issus de l'objet, dans la direction des rayons incidents 9, parviennent entre les électrodes 2 et la plaque 1 de la chambre d'ionisation principale ; il se produit alors une ionisation du xénon entre ces électrodes et cette plaque. Cette ionisation est représentée schématiquement sur la figure, par des ions Xe+ qui sont attirés par les électrodes 2, et par des électrons e- qui sont attirés par la plaque positive 1. Une ionisation se produit ainsi en regard de chacune des électrodes de la chambre d'ionisation principale grâce aux rayons X issus de l'objet, dans la direction des rayons incidents. Ces mouvements d'ions produisent respectivement dans chaque électrode, un courant 1 qui est la somme d'un courant IM, résultant de l'ionisation du gaz en regard de chacune des électrodes, sous l'effet des rayons X issus de l'objet (rayons représentés en 13 sur la figure), dans une direction correspondant à celle des rayons incidents, et d'un courant ID dit de diffusion, qui résulte de l'ionisation du gaz, en regard de chacune des électrodes à partir des rayons diffusés par l'objet (représentés en 8 sur la figure) ou par tous les obstacles matériels rencontrés par les rayons X incidents, dans des directions qui ne correspondent pas à celles des rayons X incidents. La chambre d'ionisation 7 permet de compenser ce « courant de diffusion », grâce à l'ionisation que produisent, dans cette chambre, les rayons X diffusés 8 ; cette ionisation provoque la circulation, dans les électrodes 11 de la chambre auxiliaire, d'un courant 1D qui vient se retrancher, grâce à la connexion 12, au «courant de diffusion parasite pris en compte par les électrodes de la chambre d'ionisation principale. L'étude a démontré en effet que le courant recueilli dans la chambre d'ionisation auxiliaire était représentatif du courant de diffusion recueilli dans la chambre d'ionisation principale. Ainsi, les amplificateurs 5 reliés à chacune des électrodes des chambres d'ionisation principale et auxiliaire, reçoivent un courant lm qui est effectivement le courant de mesure correspondant à l'ionisation du gaz, provoquée en regard de chacune des électrodes de la chambre d'ionisation principale par les rayons 13 issus de l'objet ou de l'organe dans les directions qui correspondent à celles des rayons incidents 9.FIG. 4 schematically represents a side view of the detector of the invention. In this view, there is a point source S, the object or the organ 0, one of the rays 9 emitted by the source S and, at the output of the object 0, the direct ray 13 coming from the object 0, in the same direction as the incident ray 9; one also distinguishes in this figure one of the scattered rays 8, coming from the object 0, in a direction different from the direction of the incident ray 9. In the figure, one shows one of the electrodes 2 of the chamber d main ionization which is connected to an amplifier 5 and which is brought to a potential close to zero and, one of the electrodes 11 of the auxiliary ionization chamber 7, which is located opposite the electrode 2 and which is separated from this electrode by the insulating plate 4. The connection 12 between the electrodes of the main and auxiliary ionization chambers has also been shown. Finally, there are shown the plates 1 and 10 of the main and auxiliary ionization chambers, brought respectively to positive and negative potentials + HT and - HT. In this figure, the sealed chamber 6 which contains the ionizable gas has not been shown in detail; the insulating plates 15, 14 support the conductive plates 1, 10 of the main and auxiliary ionization chambers. When the ionizable gas is for example xenon, the X-rays represented at 13 and which emanate from the object, in the direction of the incident rays 9, reach between the electrodes 2 and the plate 1 of the main ionization chamber; there then occurs an ionization of the xenon between these electrodes and this plate. This ionization is represented schematically in the figure, by Xe + ions which are attracted by the electrodes 2, and by e- electrons which are attracted by the positive plate 1. An ionization thus occurs opposite each of the electrodes of the main ionization chamber using X-rays from the object, in the direction of the incident rays. These ion movements produce respectively in each electrode, a current 1 which is the sum of a current I M , resulting from the ionization of the gas opposite each of the electrodes, under the effect of X-rays from the object (rays represented at 13 in the figure), in a direction corresponding to that of the incident rays, and of a so-called diffusion current I D , which results from the ionization of the gas, opposite each of the electrodes from the rays scattered by the object (represented at 8 in the figure) or by all the material obstacles encountered by the incident X-rays, in directions which do not correspond to those of the incident X-rays. The ionization chamber 7 makes it possible to compensate for this "diffusion current", thanks to the ionization produced in this chamber by the scattered X-rays 8; this ionization causes the circulation, in the electrodes 11 of the auxiliary chamber, of a current 1 D which is cut off, thanks to the connection 12, from the “parasitic diffusion current taken into account by the electrodes of the ionization chamber main. The study demonstrated that the current collected in the auxiliary ionization chamber was representative of the diffusion current collected in the main ionization chamber. Thus, the amplifiers 5 connected to each of the electrodes of the main and auxiliary ionization chambers, receive a current l m which is effectively the measurement current corresponding to the ionization of the gas, caused opposite each of the electrodes of the chamber d main ionization by the rays 13 coming from the object or the organ in the directions which correspond to those of the incident rays 9.

Les plaques et électrodes des chambres d'ionisation principale et auxiliaire sont réalisées, de préférence, sous forme d'un dépôt de cuivre sur un support isolant.The plates and electrodes of the main and auxiliary ionization chambers are preferably produced in the form of a copper deposit on an insulating support.

A titre indicatif, le nombre des cellules de chaque chambre peut être supérieur à 500, pour un angle d'ouverture du faisceau de rayons X supérieur à 40° ; dans ce cas, le pas entre chacune des électrodes de chaque chambre est de 1 mm environ. De préférence, la plaque isolante 4 qui supporte les électrodes des chambres principale et auxiliaire est située à mi-distance entre ces plaques 1 et 10, respectivement portées au potentiel positif et négatif. La distance entre ces plaques 1 et 10 est d'environ 14 mm et le temps de collection des ions est voisin de 10 ms.As an indication, the number of cells in each chamber can be greater than 500, for an opening angle of the X-ray beam greater than 40 °; in this case, the pitch between each of the electrodes of each chamber is approximately 1 mm. Preferably, the insulating plate 4 which supports the electrodes of the main and auxiliary chambers is located halfway between these plates 1 and 10, respectively brought to the positive and negative potential. The distance between these plates 1 and 10 is approximately 14 mm and the ion collection time is close to 10 ms.

Claims (9)

1. X-ray detector, adapted to detect rays, for example, that have passed through an object or organ (O) and provided by a source (S) emitting a flat beam (F) of incident X-rays in the direction of the object or organ, said beam having a large angle of divergence, and a small thickness, said detector comprising at least one closed main chamber (3) containing at least one gas that is ionizable under the influence of X-rays, and, within said chamber, a collection plate (1) for charges resulting from ionization of the gas, said plate being parallel to the plane of the incident X-ray (F) and having a first potential (+ HT), and a series of flat electrodes (2) for collection of the charges resulting from ionization of the gas, said electrodes being located facing the charge collection plate (1), in a plane parallel to the plane of the beam (F) of incident rays, on the opposite side thereof to the charge collection plate (1), each of the electrodes having its major dimension directed towards the source (S) defining a unit detection cell and providing a current (I) which is the sum of a measurement current (1M) and a diffusion current (lD), respectively proportional to the amounts of charges obtained by ionization of the adjacent gas under the effect of rays (13) emitted by the object in a direction corresponding to that of the incident rays (9), on the one hand, and under the effect of rays (8) diffused in other directions than those of the incident rays, on the other hand, and said detector being characterized in that it comprises an auxiliary ionization chamber (7), adjacent to the main ionization chamber (3), completely situated on one side of the beam (F) of incident rays and only receiving said rays (8) diffused in other directions than those of said incident rays (7), which auxiliary cell itself comprises a conductive plate (10) in a first plane parallel to that of the beam (F) and electrodes (11) located in a second plane also parallel to that of the beam (F), and each of which defines a unit detection cell providing only a diffusion current, and in that each unit detection cell of the main ionization chamber is associated with a unit detection cell of the auxiliary diffusion chamber, and in that the diffusion current provided by said last detection cell acts to compensate, in the portion of the circuit where measurement is carried out, for the diffusion current (ID) of its associated unit detection cell of the main chamber.
2. Detector according to Claim 1, characterized in that the charge collection electrodes (2) of the main ionization chamber (3) are supported on one face of an electrically-insulating plate (4), the auxiliary ionization chamber (7) containing the same ionizable gas as the main ionization chamber (3) and having a series of charge collection electrodes (11) carried on the other face of the electrically-insulating plate (4), the electrodes (11) being respectively connected to electrodes (2) of the main ionization chamber (3) and being held at the same second potential near zero, the charge collection plate (10) of the auxiliary ionization chamber (7) being parallel to the electrically-insulating plate (4), located facing the electron- collecting electrodes (11) of said auxiliary chamber, and held at a third potential (- HT) of opposite sign to the first potential.
3. Detector according to Claim 2, characterized in that the charge collection plate (1) of the main chamber (3) and the charge collection plate (10)
of the auxiliary chamber (7) are identical, the charge collection electrodes (2) of the main chamber being respectively identical to corresponding charge collection electrodes (11) of the auxiliary chamber (7).
4. Detector according to Claim 3, characterized in that the electrically-insulating plate (4) supporting the series of electrodes (2, 11) of the main chamber (3) and auxiliary chamber (7) is located half way between the charge collection plate (1) of the main chamber and the charge collection plate (11) of the auxiliary chamber.
5. Detector according to Claim 4, characterized in that the charge collection electrodes (2) of the main chamber (3) are respectively situated facing the corresponding charge collection electrodes (11) of the auxiliary chamber (7).
6. Detector according to Claim 4, characterized in that the first potential (+ HT) and the third potential (- HT) have the same absolute value.
7. Detector according to any one of Claims 1 to 6, characterized in that the ionizable gas is xenon.
8. Detector according to any one of Claims 2 to 6, characterized in that the plates (1, 10) and electrodes (2, 11) for collection of ions and electrons of the main chamber (3) and auxiliary chamber (7) comprise a layer of copper on an insulating support.
EP82400629A 1981-04-15 1982-04-06 X rays detector Expired EP0063083B1 (en)

Applications Claiming Priority (2)

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FR8107568 1981-04-15
FR8107568A FR2504278B1 (en) 1981-04-15 1981-04-15 X-RAY DETECTOR

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EP0063083B1 true EP0063083B1 (en) 1985-08-28

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US4469947A (en) 1984-09-04
JPH0335634B2 (en) 1991-05-28
FR2504278B1 (en) 1985-11-08
EP0063083A1 (en) 1982-10-20
JPS57179776A (en) 1982-11-05
FR2504278A1 (en) 1982-10-22

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